Mail mode vs message mode

[Chong Yidong <cyd@stupidchicken.com>]

If we are to switch to message-mode for Emacs 23.2, there remains
several issues that need to be resolved.  However, no one has done
anything.  If no one has fixed these issues during the next few weeks, I
think we should revert back to mail-mode.

I've taken a quick look, and the problems mainly come in three
categories.

1. Variables that are straightfowardly duplicated by message-mode.
   These variables should be either made into variable aliases (assuming
   no one would ever want to have different behaviors between
   message-mode and mail-mode):

   mail-from-style      message-from-style
   mail-envelope-from   message-envelope-from
   mail-interactive     message-interactive
   mail-yank-prefix     message-yank-prefix
   mail-signature       message-signature
   mail-signature-file  message-signature-file
   mail-default-headers message-default-headers
   mail-indentation-spaces         message-indentation-spaces
   mail-mailer-swallows-blank-line message-mailer-swallows-blank-line

   This will probably require compatibility code for XEmacs, too.

2. Hook variables.  I am not sure what to do about this, but the switch
   has bitten at least one use (Bug#3942).  Maybe we should make these
   variable aliases too:

   mail-mode-hook     message-mode-hook
   mail-citation-hook message-citation-hook
   mail-setup-hook    message-setup-hook
   mail-send-hook     message-send-hook

3. Variables that affect mail-mode but have no counterpart in message
   mode.  These need code changes:

   mail-specify-envelope-from  <???> message-sendmail-envelope-from
   mail-self-blind
   mail-yank-ignored-headers
   mail-archive-file-name
   mail-default-reply-to
   mail-default-directory
   mail-bury-selects-summary